1. Field of the Invention
This invention relates to an image processing method and apparatus. This invention particularly relates to a frequency emphasis processing method and apparatus, wherein frequency emphasis processing is carried out such that the levels of sharpness of a reproduced visible image along X and Y directions may become approximately equal to each other.
2. Description of the Prior Art
It has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image, which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness.
With the radiation image recording and reproducing systems described above, the problems are encountered in that a frequency region, which is related to a diagnosis, or the like, is not accurately illustrated in a reproduced radiation image, and in that the contrast and the sharpness of the reproduced radiation image are low and thus a visible image, which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness, cannot be obtained. In view of the above circumstances, in U.S. Pat. No. 4,315,318, the applicant proposed an image processing method, which can improve the efficiency and accuracy of a diagnosis, or the like, by emphasizing the frequency components, which are not lower than super-low frequency and are related to a diagnosis, or the like.
With the proposed image processing method, a digital image signal, which represents picture element values of picture elements arrayed along X and Y directions of an image, is subjected to operation processing carried out with the formula EQU Q'=Qorg+.beta.(Qorg-Qus)
wherein Qorg represents the original image signal representing each picture element, Qus represents the unsharp mask signal corresponding to the super-low frequency with respect to the picture element, and .beta. represents the emphasis coefficient.
The unsharp mask signal Qus corresponding to the super-low frequency is the signal representing the density of each picture element of an unsharp image, which has been obtained by rendering the original image unsharp such that the unsharp image may contain only the frequency components lower than the super-low frequency.
When the radiation image is reproduced from the new image signal Q', which has been obtained from the aforesaid operation processing, a reproduced visible radiation image can be obtained, in which the frequency components, that are not lower than the super-low frequency and are related to a diagnosis, or the like, have been emphasized and which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness.
In such cases, as an image reproducing apparatus, an apparatus is ordinarily used which is constituted to reproduce an image by operations for scanning along X and Y directions. Examples of such image reproducing apparatuses include a cathode ray tube (CRT) display device, a thermal recording apparatus utilizing a thermal head, and a light beam scanning recording apparatus. However, it has heretofore been found that, in cases where the image reproducing apparatus of this type is used, the problems occur in that the levels of sharpness of the reproduced visible image along X and Y directions become different from each other. It is considered that the aforesaid problems occur due to a difference between the responses of the image reproducing apparatus along the X and Y directions.
The sharpness of a reproduced image can be adjusted by changing the emphasis coefficient .beta. in the aforesaid operation processing. Therefore, the emphasis coefficient .beta. may be set to be comparatively large, and the sharpness along the direction, in which the response is low, may thereby be kept high. However, in such cases, it often occurs that the frequency emphasis becomes excessive with respect to the direction, in which the response is high, and an artifact occurs in the reproduced image.